Mobile handset applications demand an LDO architecture that works down to very low input voltage (for example, 1.4V) still providing good power supply rejection ratio (PSSR) and transient response. One conventional technique to achieve this is to use a charge pump 110 to boost a gate of a pass transistor 115 as illustrated in FIG. 1. The constraint on an NMOS based power device (pass transistor) design is that the gate of the power device needs to be enhanced higher than the output voltage. Under very high output voltage (VOUT) applications, it would demand that the power supply to be higher than VOUT. To solve this problem, the conventional approaches charge the gate of the pass transistor 115 to a higher voltage by pumping extra charge to the gate. One conventional charge pumping technique is to have a series coupling capacitor 135 isolating the gate of the pass transistor 115 from the amplifier 105 and to use this capacitance as a level shifter. This capacitor 135 is charged to a predetermined voltage which is controlled by a servo amplifier 140 whose supply alone is boosted.
However, the conventional charge pump technique results in higher output noise due to ripple leak from the switching circuit. Few additional problems with the conventional charge pump technique are explained below. The servo amplifier 140 needs additional charge pump on the supply which is constantly drained by the servo amplifier's 140 quiescent current. This not only requires additional higher value capacitor but also needs to be switched at high enough frequency that the voltage change at the input of the servo amplifier 140 is not so high that it results in additional problems. The servo amplifier 140 also needs to have very high PSRR at the frequency of switching so that that input ripple does not appear as a ripple at the gate, which cannot be filtered, especially at higher load current where the NMOS pass transistor 115 has very high bandwidth. The sudden changes in the line and load of the servo amplifier 140 will result in either over charging the output capacitance in line or losing charge on the capacitor 135 as the load change has coupled through the CGS and discharged the output capacitor 125 which could not be replenished quickly enough by the servo amplifier 140. Additionally, the servo amplifier 140 has to be a wideband amplifier which requires additional power. The internal switches of the charge pump 110 have to be carefully implemented to avoid dumping charge on VIN, which in some cases is not ideal.